1. Field of the Invention
The present invention relates to an image recording apparatus such as a printer and the like.
2. Related Background Art
Some image recording apparatuses such as printers have a recording head as a recording means. In such image recording apparatuses, in order to obtain a good recorded image, a distance between a surface of a recording head opposed to a recording medium (for example, a surface in which ink discharge openings are formed in an ink jet recording head) and the recording medium must be kept constant. However, a thickness of the recording medium used in the image recording apparatus varies with kinds of the recording media (normal sheet, thick sheet, envelope and the like). It is desirable that the above-mentioned distance is kept constant even when the thickness of the recording medium is changed.
FIGS. 15 and 16 show a conventional image recording apparatus in which the above-mentioned problem is taken into consideration.
A main sheet feed roller 1 is constituted by a core portion 1a and a convey portion 1b having an outer peripheral surface of predetermined coefficient of friction, and a shaft portion (not shown) formed integrally with the core portion 1a is rotatably supported by left and right inner walls (not shown) of a frame 2. A sub sheet feed roller 3 is rotatably supported by a platen 4 and has an outer peripheral surface of predetermined coefficient of friction.
A pressure plate 22 is pivotally supported by the inner walls (not shown) of the frame 2 and is biased upwardly by a pressure plate spring 23 so that it can be rocked at a predetermined timing. A sheet stack P resting on the pressure plate 22 is urged against the convey portion 1b of the main sheet feed roller 1 so that only an uppermost sheet (recording medium) can be supplied.
Spurs 5 each formed from a thin plate having teeth at its outer periphery are supported by a spur holder 6 via an elastic shaft 5a formed from an elastic body such as a spring and are urged against the sub sheet feed roller 3 with predetermined pressure by the elastic shaft 5a so that the spurs are rotatingly driven by rotation of the sub sheet feed roller 3.
An idle roller 7 is urged against the main sheet feed roller 1 and the sub sheet feed roller 3 substantially along a tangential direction by means of an idle roller spring 8 secured to a predetermined position on the platen 4 so that a rotational force of the main sheet feed roller 1 can be frictionally transmitted to the sub sheet feed roller 3. Auxiliary rollers 26 are driven rollers and are rotatably urged against the convey portion 1b by an urging spring 27 disposed along a sheet guide surface 2e so that the recording medium P to be conveyed is urged against the convey portion 1b to thereby enhance a conveying force of the convey portion 1b for the recording medium.
Left and right outer plates (not shown) are provided outside of the frame 2, and a central U-shaped sheet guide surface 2e and left and right inner walls (not shown) acting as lateral guides are provided. A guide shaft 10 is supported by the left and right outer plates, and a carrier 12 is slidably mounted on the guide shaft. A recording head 9 is of a so-called disposable type having a recording portion and an ink tank and is detachably mounted on the carrier 12.
The carrier 12 is connected to a timing belt (not shown) wound around and mounted on a motor pulley (not shown) of a carrier motor (not shown) provided near the right outer plate of the frame 2 and a tension pulley (not shown) provided near the left outer plate of the frame 2. As a result, when the carrier motor is driven, a driving force of the carrier motor is transmitted to the carrier 12 through the timing belt to scan the carrier along the guide shaft 10 and an auxiliary guide 11. A slide portion 12a of the carrier 12 is contacted with a slide surface 19a of a pressure plate 19 so that, when the carrier is scanned, it is reciprocally shifted while sliding on the slide surface.
The pressure plate 19 is formed from a plate-shaped member also acting as a sheet guide and has a base end 19b secured to the frame 2 and a tip end for rotatably holding needle rollers 20. The needle rollers 20 are urged against the outer peripheral surface of the main sheet feed roller 1 and are rotatingly driven by the rotation of the main sheet feed roller 1. The pressure plate 19 has flexibility along a direction (longitudinal direction) substantially perpendicular to a sheet conveying direction, and the needle rollers 20 are urged against the convey portion 1b at respective positions with predetermined pressure by means of urging springs 21 provided along the longitudinal direction.
The platen 4 is secured to the frame 2 at a predetermined position. The platen 4 is disposed in correspondence to an upper part of the main sheet feed roller 1 and has a recording surface 4a opposed to a nozzle surface 9a of the recording head 9 and a bearing portion (not shown) for rotatingly supporting the sub sheet feed roller 3.
With the above-mentioned arrangement, while the recording medium P is being conveyed in contact with or in the vicinity of the recording surface 4a of the platen 4, an image is recorded on the recording medium by the recording head 9. In this case, when the recording medium P is a relatively thin sheet such as a normal sheet, as shown in FIG. 16, the recording medium P on the recording surface 4a is substantially in parallel with the nozzle surface 9a so that a distance (.DELTA.d-1) therebetween is substantially constant so long as dimensions of constructural elements are correct to thereby achieve the good recording.
However, as shown in FIG. 17, when the recording medium P to be conveyed is a relatively thick recording medium such as a thick sheet or an envelope, the needle rollers 20 are lifted upwardly by a distance corresponding to a thickness of the recording medium and the slide surface 19a of the pressure plate 19 is also shifted accordingly. Thus, the carrier 12 slidingly contacted with the slide surface 19a is rotated around the guide shaft 10, so that the nozzle surface 9a of the recording head 9 mounted on the carrier 12 is retarded from the normal position. As a result, the distance between the nozzle surface 9a and the recording medium P becomes (.DELTA.d-2).
In this image recording apparatus, the guide shaft 10 around which the carrier 12 is rotated, the slide portion 12a and the pressure plate 19 are arranged so that the distances (.DELTA.d-1) and (.DELTA.d-2) become the same as each other, so that the distance between the nozzle surface 9a and the recording medium P is automatically kept constant in accordance with the thickness of the recording medium P.
However, in the above-mentioned conventional image recording apparatus, since the platen 4 is secured to the frame 2 at the predetermined position, if the deflection accuracy of the main sheet feed roller 1 is not correct, the needle rollers 20 contacted with the convey portion 1b are shifted in accordance with the deflection of the main sheet feed roller 1. Thus, the pressure plate 19 and the carrier 12 are also shifted, so that the distance between the nozzle surface 9a and the recording medium P is changed during the recording operation for the single recording medium P. Thus, a good recorded image cannot be obtained.
That is to say, in the above-mentioned conventional image recording apparatus, the U-shaped convey path is used as the sheet passing passage, and, when the recording medium P is passed through such a U-shaped convey path, by providing the main sheet feed roller having a large diameter and the convey path having gentle and large curvature, even the recording medium having great resiliency can be passed through the convey path without generating any curl. In order to manufacture the main sheet feed roller 1 having the large diameter with light-weight and low cost, generally, the core portion of the roller is molded from resin and a rubber ring having predetermined coefficient of friction is fitted on the core portion.
However, when the resin molded core portion is used, a roller having the same high deflection accuracy as a metallic core portion cannot be obtained, and the deflection of the roller is always generated to thereby vary the distance between the nozzle surface and the recording medium.